This invention relates to a digital signal processing device for matching the sampling frequency of an input digital signal to an internal sampling frequency.
If, in a digital signal processing device in which signal processing is performed on digital signals having a pre-set sampling frequency, a signal is entered which has been processed with a sampling frequency different from the pre-set sampling frequency employed in the device, there is typically no alternative but to convert the sampling frequency with the aid of a sampling rate converter in order to admit the input signals into the device.
The constitution of a sampling rate converter for of converting the sampling frequency is schematically shown in FIG. 1. It is assumed that the signal sampled at the sampling frequency FS.sub.44.1 of 44.1 kHz, as in the case of a compact disc (CD), is to be recorded on a tape using a digital tape recorder (DAT) having the sampling frequency FS.sub.48 of 48 kHz. That is, since the playback signal from the CD is the signal with the sampling frequency FS.sub.44.1 with which the signal is recorded on the disc, the sampling frequency FS.sub.44.1 of the playback signal from the disc needs to be converted into the sampling frequency FS.sub.48 for DAT if the signal is to be recorded on the DAT.
Referring to FIG. 1, an input signal reproduced from the CD, with the sampling frequency FS.sub.44.1 is supplied to an input terminal 100 and thence to a sampling rate converter 101. The sampling rate converter is fed via a terminal 103 with an internal frequency signal corresponding to the sampling frequency FS.sub.48 of the DAT. The sampling rate converter 101 over-samples the input signal with the sampling frequency FS.sub.44.1 and interpolates necessary data by data interpolation. The resulting signal is down-sampled based upon the internal frequency signal for conversion to the signal with the sampling frequency FS.sub.48. This down-sampled frequency-converted signal is fed via a terminal 102 to the DAT.
By this sampling frequency conversion, the reproduced signal from the CD, recorded on the tape by DAT, retains the inherent pitch (interval) with which the signal was recorded on the CD.
Meanwhile, if it is assumed that the tape running speed on the DAT slightly deviates from a pre-set speed, as when the tape is run at a slightly higher or lower rate, the data transfer rate is also slightly deviates from the pre-set rate. If the output signal from the DAT is supplied to an external equipment, such as another DAT, the external equipment determines that the sampling frequency has deviated.
If the sampling frequency is matched using the sampling rate converter, the interval of the resulting signal deviated from the pitch with which the signal was inherently recorded on the DAT. This is tantamount to slightly faster or slower rotation of an analog tape recorder.
As an illustrative example, a commercial music piece which should be 30 seconds long becomes 31 seconds long on recording, and the tape so recorded is run (rotates) at a slightly faster speed for reproducing the music piece within 30 seconds.
If an external equipment operating with the inherent sampling frequency with which the music piece was recorded should receive the signal reproduced at the slightly faster speed, such external equipment converts the sampling frequency using the above-mentioned sampling rate converter in order to admit the music piece. In this case, since the signal recorded in 31 seconds is reproduced in 30 seconds, the pitch (interval) is higher than the inherent pitch.